The invention relates to well screens, and particularly to plastic well screens which are sometimes used in preference to conventional welded metal wire screens of the type disclosed in Johnson U.S. Pat. No. 2,046,458 to achieve lower cost or to provide a particularly non-corrosive material compatibility with the fluid being collected.
Plastic screens can be made by wrapping plastic wire around rods and welding it with ultrasonic energy or solvents, for example, but low cost plastic well screens are generally manufactured from plastic pipe or custom designed tubular plastic extrusions. The custom extrusions usually have ribs on the inside for longitudinal strength. External slots may be cut with a moving saw blade, by rotating the extrusion and using a stationary cutting tool, or by a combination of these. This results in a straight sided slot, and, because of the geometry of the saw blade or because of the strengthening ribs, a relatively low percentage of open area. Cutting internal slots with a cutter having tapered sides would provide slots having tapered walls and has been done in a metal screen having longitudinal external ribs. However, internal cutting is very difficult and would also considerably reduce the open area of the slots due to the cutter geometry. Small slot screens are somewhat difficult to fabricate because the slot is sized by the width of the cutting tool. In the 0.010" slot range, cutting tools are weak and difficult to control. Also, externally sawn slots, having straight sides, do not provide the self-clearing or non-clogging advantage of slots with tapered sidewalls.